This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Recent developments of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) facilitate accessing local Internet Protocol (IP)-based services in various places, such as at home, office, or public hot spots, or even in outdoor environments. One of the important use cases for the local IP access and local connectivity involves a so-called D2D communication mode, wherein UEs in close proximity (typically less than a few tens of meters, but sometimes up to a few hundred meters) of each other communicate with each other directly.
Because D2D UEs are much closer to each other than cellular UEs that have to communicate via at least one cellular access point (e.g., an evolved NodeB (eNB)), the D2D communication enables a number of potential gains over the traditional cellular technique, including capacity gain, peak rate gain, and latency gain.
The capacity gain may be achieved, for example, by reusing radio resources (e.g., Orthogonal Frequency Division Multiplexing (OFDM) resource blocks) between D2D and cellular communications and by reducing the number of links between UEs from two to one and accordingly reducing the radio resources required for one link. The peak rate gain directly results from the relatively short distance between D2D UEs and the potentially favorable propagation condition therebetween. The latency gain is also a direct result of the single relatively short link between D2D UEs.
FIG. 1 illustrates an example of a mixed cellular and D2D network 100, wherein UE 101 is a cellular UE which communicates via an eNB 103 using a cellular link 105, whereas UE 108 and UE 110 are D2D UEs which communicate with each other directly using a D2D UE link 115. In such a mixed cellular and D2D network 100, D2D communications share radio resources with cellular communications. A Time Division Duplex (TDD) is used as the duplex scheme for the bi-directional D2D communications in FIG. 1.
A pure cellular system may comprise only the UE 101 and the eNB 103 in FIG. 1. It does not comprise the UE 108 and UE 110 which communicate using the D2D UE link 115. For a pure cellular system using a TDD scheme to work properly, a Guard Period (GP) is configured at the transition between DownLink (DL) communications and UpLink (UL) communications, as illustrated in FIG. 2. A GP may be described as a time interval where no radio transmission may occur. The purpose of the GP is to protect adjacent data from transmission overlap due to propagation time of the data, i.e. to avoid interference. A GP length is related to a cell size. More specific, the GP is larger than twice the transmission delay for a signal transmitted between the eNB and the UE 101, i.e. the delay for a transmission from the eNB 103 to the UE 101 or the delay for the transmission from the UE 101 to the eNB 103. The GP exists only in TDD system which is used to handle the transmission delay from the eNB 103 to the UE 101 (i.e. DL) and the Timing Advance (TA) of the UE 101 to transmit (i.e. UL). Thus, the GP is between a downlink and an uplink transition. With the GP positioned between the Downlink Pilot TimeSlot (DwPTS) and the Uplink Pilot TimeSlot (UpPTS), the DL data transmitted from the eNB 103 can be fully received by the UE 101 before the UL data is sent from the UE to the eNB 103 with a TA. A TA is used by the UE 101 to transmit data. Different UEs have different TAs so that their signals can be time aligned at the eNB 103. The transmission delay seen in FIG. 2 is the delay of the data transmitted from the eNB 103 to the UE 101.
The time instance when the DL data is transmitted from the eNB 103 is indicated as eNB TX in FIG. 2 and the time instance when the DL data is received by the UE 101 is indicated as UE RX in FIG. 2. The time instance when the UL data is transmitted from the UE 101 to the eNB 103 is indicated as UE TX and the time instance when the UL data is received by the eNB 103 is indicated as eNB RX in FIG. 2. TX refers to transmitting and RX refers to receiving.
The DwPTS mentioned above is a field which carries synchronization, user data and the downlink control channel for transmitting scheduling and control information. The UpPTS is a field which is used for transmitting a physical random access channel and a sounding reference signal.
The term DL mentioned above refers to communication in the direction from an eNB 103 to a UE 101, and the term UL refers to communication in the direction from a UE 101 to an eNB 103.
Transmissions in the mixed cellular and D2D network may utilize a frame and subframe structure when managing the data that needs to be transmitted. A frame may be divided into a number of subframes. A subframe may be of a certain length, it may comprise a number of slots etc. A cellular subframe may be a subframe used to carry data between the UE and the eNB. A D2D subframe may be a subframe used to carry data between two UE's, i.e. between D2D UEs. A subframe may comprise at least one OFDM symbol. A D2D subframe transmitted by a UE (to another UE) is referred to as a D2D TX subframe. A D2D subframe received by a UE (from another UE) is referred to as a D2D RX subframe.
In the mixed cellular and D2D network 100, there exist other communication transitions than the DL/UL transition. From the perspective of one UE, the communication transition may additionally occur between a cellular subframe and a D2D TX subframe, between a cellular subframe and a D2D RX subframe, or between a D2D TX subframe and a D2D RX subframe. At these transitions, overlap as described above might as well happen, which can affect data transmission/reception.